The regular price of a child's entry ticket to a water park is $9 less than that for an adult's. the park offers half off all entry tickets during the off-peak season. the sandlers paid a total of $132 for 1 adult ticket and 4 child's tickets to the water park during the off-peak season. the following equation represents this situation, where x represents the regular price of an adult ticket:

132 = 1/2x + 2(x − 9)

what is the regular price of a child's ticket?

$60
$57
$51
$42